The present invention relates to amorphous silicon sensors, and in particular, it relates to an amorphous silicon Schottky barrier device having a metal annealed layer.
A semiconductor X-ray sensor is a device for converting X-ray energy to an electrical signal. In general, such sensors sense either the collection of the charge generated by absorption of the incident X-ray photons in the semiconductor itself or the collection of the charge generated in the semiconductor by absorption of visible light photons which are produced by a phosphor or scintillation element excited by the X-rays.
The absorption and conversion of X-rays into light photons by a phosphor screen followed by the conversion of the light photons into electrical charge in the semiconductor device is the more efficient process and is generally chosen whenever the statistical accuracy of the number of photons is preferred to the accuracy in the energy or time resolution of the impinging radiation. This approach is suited for Computer Tomography and Electronic Radiography where the image is close to the quantum limit because of the very low dosage requirement.
Beerlage et al in an article entitled "Digital Slot Radiography Based on a Linear X-Ray Image Intensifier and Two-Dimensional Image Sensors" describe the use of digital radiography. Digital radiography is useful in situations where a large area needs to be imaged. However, the sensors used in digital radiography are expensive hybrid assemblies of discrete single crystal devices and do not allow high resolution and high quality imaging.
A phosphor layer coated on metal electrodes of a junction field effect transistor (JFET) that is deposited on top of amorphous silicon Schottky devices as described in European Patent Application 189,710. A charge is injected and trapped through the source and drain electrodes into the potential well close to the gate electrode. This charge inhibits the conduction between the source and drain electrodes and can be neutralized by charge generation produced by the light from the phosphorus screen. Current between the source and drain electrodes is taken as the signal. In European Patent Application 163,956, an X-ray sensor having an amorphous silicon p-i-n structure is described. The sensor structure is conventional and utilizes single transistors or single diodes as blocking elements in a pixel.
More recently an amorphous silicon X-ray sensor has been described in Japanese laid-open patent applications Nos. 61-196582, 61-196572, 61-196571, and 61-196570. The amorphous silicon X-ray sensor described in these Japanese patent applications is fabricated on glass substrates by the decomposition of SiH.sub.4 /H.sub.2 mixtures containing the desired amounts of CH.sub.4, B.sub.2 H.sub.6, PH.sub.3 in a RF glow discharge deposition system to produce amorphous silicon layers (a-Si:H) with various levels of doping. The structure of the sensor is a glass substrate /ITO (Indium Tin Oxide) layer /p-type a-SiC:H layer /i-type a-Si:H layer /microcrystalline n-type a-Si:H layer /Al layer. The thickness of the p-type and n-type layers is 120-150 Angstrom and 500 Angstrom, respectively. A layer of ZnS (Ni doped) or CaWO4 phosphor is coated on the front surface of the glass substrate. When X-rays are incident on the phosphor, green or blue light is emitted. The light then penetrates the glass substrate and finally enters the p-i-n sensor. However, systems made from this sensor suffer from image contrast losses and are limited in spatial resolution and dynamic range.
Amorphous silicon Schottky barrier diodes and photosensors are well known in the art. Solar cells made using amorphous silicon (a-Si) produced by RF glow discharge in silane were found to contain a much smaller density of defects than samples previously made by direct evaporation or sputtering (D. E. Carlson and C. R. Wronski, "Amorphous Silicon Solar Cell", App. Phys. Lett. Vol. 28, 671 (1976)). Electrical properties of Schottky barriers formed between undoped amorphous silicon and metals such as chromium, palladium, rhodium, and platinum have been studied (C. R. Wronski and D. E. Carlson, "Surface States and Barrier Heights of Metal-Amorphous Silicon Schottky Barriers", Solid State Comm. Vol. 23, 421 (1977)).
Schottky diodes formed between platinum and undoped a-Si:H produced by RF glow discharge in silane/hydrogen mixtures, are shown (A. Deneuville and M. H. Brodsky, "Influence of Preparation Conditions on Forward-Bias Currents of Amorphous Silicon Schottky Diodes", J. Appl. Phys. Vol. 50, 1414 (1979)) to have improved characteristics (closer to ideality) on post deposition annealing at a temperature equal to the deposition temperature. No details of the atmosphere are given explicitly. The formation of metal silicide at the metal/a-Si:H interfaces on post-deposition annealing is studied in M. J. Thompson et al, "Silicide Formation in Pd-a-Si:H Schottky Barriers", Appl. Phys. Lett. Vol. 39, 274 (1981). Annealing was under vacuum at 180.degree. C. for 15 min. Ideality factors improved and became stable on annealing.
In R. J. Nemanich et al, "Initial Reactions at the Interface of Pt and Amorphous Silicon", J. Vac. Sci. detailed studies were described of the Pt silicide layers formed between a thin electron beam deposited Pt layer and a-Si:H layer in a Schottky barrier diode. No photo response characteristics were examined but the backward bias (-1 V) current densities were given as about 10.sup.-10 A. /cm.sup.2 as-formed and 10.sup.-11 A. /cm.sup.2 after vacuum annealing at about 200.degree. C.
In R. J. Nemanich in "Semiconductors and Semimetals", Vol. 21, Part C, page 376, edited by Jacques Pankove, Academic Press 1984, metal silicides are reported to occur at junctions with a-Si:H with the following metals, chromium, nickel, palladium, and platinum.